Composting is a complex and dynamic biological process, resulting from the activities of a succession of mixed populations of microorganisms (and perhaps also some higher organisms). The activity of each microbial group is often limited by narrow and exacting environmental requirements (mostly temperature and oxygen tension). At the early stages of composting mesophilic bacteria and fungi multiply rapidly, by metabolising easily degradable compounds. Bacterial populations of up to 108-109 g-1 of moist compost have been reported (Abouelwafa et al., 2008; Raut et al., 2008; Saludes et al., 2008; Heerden et al., 2002; Andrews et al., 1994; Davis et al., 1992; Strom, 1985). As metabolism continues the temperature rises to thermophilic range of up to 60oC and higher. Mesophilic organisms, including most of the non-spore forming bacteria are rapidly inactivated, (or at least cease to be active at this temperature (Droffner et al., 1995ab)).
During the thermophilic stage, most of the biological activity is due to spore forming thermophiles (Elango et al., 2008; Fujio and Kume, 1991) and actinomycetes (Durak and Ozturk, 1993; Singh et al., 1991). The majority of the microbial activity at this stage is directed towards the breakdown of high molecular weight polymers, including cellulose, hemicelluloses, proteins, lipids and starch. As readily degradable compounds are exhausted, the temperature begins to drop leading to the re-establishment of mesophiles in the final or maturation stage. At this stage, actinomycetes and fungi continue to breakdown the high molecular weight polysaccharides and formation of humic acid takes place.
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